Fuses are commonly used in integrated circuits to provide redundancy and programming capabilities. To increase yield in integrated circuits such as memory chips, it is common to include redundant memory cells on the memory chips. If a memory circuit is found to be defective or is not needed, the fuse may be blown thereby activating or deactivating the redundant memory cells. Another common practice is to utilize fuses to program or customize integrated circuits for a particular application or customer. In this manner, the same chip may be produced and customized for individual customers by programming the fuses after fabrication, thereby reducing the fabrication costs.
Typically, fuses comprise a conductive link that may be blown or ruptured to prevent current from flowing. In one particular type of fuse, the conductive link is formed of a metal, such as aluminum or copper, and blown by a laser. The use of the laser, however, requires complicated processing steps and expensive laser equipment.
Another type of fuse involves the use of an electrical fuse, which may be blown by passing an electrical current of sufficient magnitude through the selected fuses for a sufficient period of time to alter the electrical properties of the link, generally increasing the resistance of the link. A common design for such a fuse comprises a cathode and an anode interconnected by a thinner fuse link. Such a structure is commonly formed of doped polysilicon or undoped/doped polysilicon having a silicided surface.
To blow the fuse, a sufficiently high current is applied to the link causing high current concentrations or “current crowding” where the dimensions of the fuse are reduced in the link. The current crowding causes silicide agglomeration or melting of the link, increasing the resistance of the link. A sensing circuit is then able to sense the amount of resistance to determine the state of the fuse.
In an attempt to reduce the amount of current and the time period required to blow the fuse, further attempts have incorporated a p-n junction diode in conjunction with the silicided layer. In a typical design, the cathode, anode, and fuse link are formed of a polysilicon material. The cathode of the fuse is doped with p-type impurities, and the fuse link and the anode are doped with n-type impurities. The junction of the p-type cathode and the n-type fuse link forms a p-n junction diode. The surface of the polysilicon at the p-n junction diode is silicided.
To blow the fuse, the cathode is negatively biased and the anode is positively biased causing a reverse bias to be applied to the p-n junction diode. Because the p-n junction diode does not allow the current to flow in this configuration, the current flows through the silicided layer. The current crowding in the silicide layer over the p-n junction causes silicide migration or melting of the link, thereby blowing the fuse. Thereafter, the link is a high-resistance path and allows sensing circuits to detect the blown state of the fuse.